Printed bandpass filter for a double conversion tuner

ABSTRACT

A printed bandpass filter is mounted on a precision substrate to eliminate the need for post-fabrication tuning. The filter input is capacitively coupled to a series of quarter wavelength resonators and the filter output. The quarter wavelength resonators are printed as spirals to reduce filter size. The resonators define the bandpass characteristics of the filter. The filter also weakly couples the input signal to the filter output in a manner to cancel the signal image. Mechanical clips mitigate thermal stress on solder connections when the precision substrate mounted on a second printed circuit board.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to radio frequency signalfilters, and more specifically to printed circuit bandpass filters.

[0003] 2. Background Art

[0004] Television tuners can be classified by the type of circuit usedto select the desired television channel. The predominant circuitarchitectures in use today are single-conversion and double-conversiontelevision tuners.

[0005] Single conversion tuners usually require preselection filtering.The preselector must be a tracking bandpass filter in order to rejectthe image channel, which occurs at twice the intermediate frequency (IF)away from the desired television channel frequency. Tracking filtersrequire expensive manual tuning during the assembly process. Trackingfilters can have significant variations in amplitude response over thedesired television channel bandwidth. These variations are undesirablein both analog and digital television systems. Tracking filters are alsoparticularly difficult to implement at the upper end of the televisionband, where the difference between the desired television channelfrequency and the image frequency is a small fraction of the desiredtelevision frequency. Removing the image channel, under theseconditions, requires a bandpass filter with high selectivity.

[0006] Double-conversion tuners convert the incoming television signalto a high IF, where most of the out-of-band signals are removed by anarrow bandpass filter. This high IF bandpass filter is usuallyimplemented as either a surface acoustic wave (SAW) filter or amanually-tuned LC filter. The high IF bandpass filter passes a fewchannels, out of more than 100 channels in the television band. A secondconversion brings this relatively narrowband signal composed of a fewchannels down to the standard television IF at about 40 MHz. A secondSAW or LC filter eliminates the remaining undesired channels.

[0007] There are several advantages to the double-conversion tuner.First, a tracking filter is not required for image rejection. It iseasier to obtain a high level of image rejection with thedouble-conversion approach, because a fixed surface acoustic wave and afixed LC filter can be much more selective than a tracking LC filter.Second, by tuning coarsely with the first broad tuning local oscillator,and fine-tuning with the second narrow tuning local oscillator, thenecessary complexity of both phase-locked loops can be substantiallyreduced.

[0008] The high IF bandpass filter, which is usually centered a fewhundred megahertz above the upper limit of the television band, must bewide enough to pass the desired television channel under all conditionsof center-frequency manufacturing tolerance; center-frequencytemperature and other environmental drift; and the variability of thehigh IF center frequency due to coarseness in tuning the first localoscillator.

[0009] Each of the described high IF filters have disadvantages. A fixedLC filter is composed of lumped element capacitors and inductors.Variations in the values of these components and variations in thecharacteristics of the underlying substrate cause a shift in thefilter's characteristics, center frequency, bandwidth, etc., duringfabrication. To compensate, lumped element filters must be tuned afterfabrication. Tuning raises the cost and complexity of the filterassembly process.

[0010] Surface acoustic wave (SAW) filters do not require postfabrication tuning. However, SAW filters are relatively expensive andcostly to integrate into new circuit designs, and cannot be fabricatedat generic printed circuit board facilities.

[0011] What is needed is a passive bandpass filter that exhibits highselectivity, low input return loss, low insertion loss, and good imagechannel rejection. This new filter should also be inexpensive, capableof manufacture at generic printed circuit board facilities and notrequire post fabrication tuning.

BRIEF SUMMARY OF THE INVENTION

[0012] The invention is a printed bandpass filter comprising an input, abypass line, an output, and a plurality of resonators. The input,resonators, and output are coupled to provide a desired passband. Thebypass line is weakly coupled to the input and the output to improveimage rejection of the input signal. The filter is printed on aprecision substrate that can be mounted on a motherboard. Use of theprecision substrate mitigates variations in the filter and eliminatesthe need for post fabrication tuning. The resonators are quarterwavelength transmission lines laid out in a pattern and coupled toground. The resonators become open circuits at a resonant frequency.

BRIEF DESCRIPTION OF THE FIGURES

[0013] The present invention is described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the leftmostdigit of a reference number identifies the drawing in which thereference number first appears.

[0014]FIG. 1 illustrates a double conversion tuner.

[0015]FIG. 2 illustrates a schematic of the bandpass filter according tothe present invention.

[0016]FIG. 3A illustrates a printed circuit embodiment of the bandpassfilter.

[0017]FIG. 3B illustrates a printed bandpass filter for use indifferential signal applications.

[0018]FIG. 3C illustrates details of a bandpass filter printed on aprecision substrate.

[0019]FIG. 4A illustrates the connections for attaching the printedfilter assembly to a second printed circuit board.

[0020]FIG. 4B illustrated details of connecting the printed filterassembly to a second printed circuit board.

[0021]FIG. 5 illustrates the apparatus used to mount a printed bandpassfilter assembly to another printed circuit board.

[0022]FIG. 6 is a flow chart illustrating the steps used in designing aprinted bandpass filter.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Example Tuner Application

[0024] Before describing the invention in detail, it is useful todescribe an example tuner environment for the invention. The printedbandpass filter invention is not limited to the tuner environment thatis described herein, as the bandpass filter invention is applicable toother tuner and non-tuner applications as will be understood to thoseskilled in the relevant arts based on the discussions given herein.

[0025]FIG. 1 illustrates a double conversion tuner 100. The doubleconversion tuner 100 comprises a variable gain low noise amplifier 102coupled to a first mixer 106 and a tuner input 101. The first mixer 106is also coupled to a first local oscillator 104. A high IF bandpassfilter 108 is coupled to the first mixer 106 and a second mixer 110. Thesecond mixer 110 is coupled to a second local oscillator 112 and asecond IF bandpass filter 114. A variable gain amplifier 116 is coupledto the second IF bandpass filter 114 and the tuner output 117.

[0026] The low noise amplifier 102 amplifies a radio frequency (RF)signal 120 present at the tuner input 101 and sends it to the firstmixer 106. In an embodiment, the RF signal 120 is a television signalbetween approximately 50 and 850 megahertz. The first mixer 106 combinesthe RF signal 120 with the output of the first local oscillator 104 andoutputs a high IF signal 130. The high IF signal 130 comprises a signalat approximately 1220 megahertz and an image channel component atapproximately 1132 megahertz. The high IF bandpass filter 108 is abandpass filter with its passband centered at approximately 1220megahertz. The high IF signal 130 is filtered by the high IF bandpassfilter 108. The high IF bandpass filter 108 removes the image channelcomponent and most of the undesired television channels. Afterfiltering, the high IF signal 130 becomes a high filtered IF signal 135.The high filtered IF signal 135 is mixed with the output of the secondlocal oscillator 112 in the second mixer 110 to become a low IF signal140. The low IF signal 140 comprises the television channels passed bythe high IF bandpass filter 108 reduced in frequency to approximately 44megahertz. The low IF signal 140, at approximately 44 megahertz, is astandard television IF used in the United States. An embodiment of thedouble conversion tuner 100, for use in Europe, produces the low IFfinal signal 140 at approximately 36 megahertz.

[0027] The second IF bandpass filter 114 removes the undesiredtelevision channels from the low IF signal 140 and outputs an IF signal145. The IF signal 145 comprises the desired television channel and issent to the variable gain amplifier 116 for amplification, then to thetuner output 117.

[0028] The Printed Bandpass Filter

[0029] This invention is a novel implementation of the high IF bandpassfilter 108 using quarter wavelength resonators, coupled in parallel witha bypass line, and printed on a precision substrate.

[0030]FIG. 2 illustrates an electrical schematic of a bandpass filter200 according to the present invention. Referring to FIG. 2, thebandpass filter 200 comprises an input capacitor 204 coupled to a filterinput 202, a bypass line input coupler 208, a first resonator 206, and afirst intercoupler 210. The first intercoupler 210 is coupled to asecond intercoupler 216, and a second resonant element 214. The secondintercoupler 216 is coupled to a bypass output coupler 218, a thirdresonator 220, and an output capacitor 222. A bypass line 212 is coupledto the bypass line input coupler 208 and the bypass line output coupler218. The output capacitor 222 is coupled to the filter output 224. Thefirst resonator 206, the second resonator 214, and the third resonator220 are coupled to a ground 226.

[0031] The ground 226 is located beneath the bandpass filter 200, theprinted metal traces comprising the input capacitor 204, first resonator206, first intercoupler 210, second resonator 214, second intercoupler216, third resonator 220, output capacitor 222, bypass line inputcoupler 208, bypass line output coupler 218, and bypass line 212 aremicrostrip transmission lines. Other equivalent transmission lines couldbe used. In one embodiment, the input capacitor 204 and the outputcapacitor 222 are printed finger capacitors. Printed finger capacitorsare used to provide stronger capacitive coupling than is possible withtransmission lines. The finger capacitors are simpler and less expensivethan discreet surface mount capacitors and can be used on a single layerprinted circuit board. The finger capacitors provide the necessarycoupling capacitance without increasing the cost or complexity of thebandpass filter 200. Capacitors, other than finger, could be utilized aswould be understood by those skilled in the art.

[0032] Adjusting the lengths of each resonator along with the inductiveand capacitive characteristics of the coupled transmission lines, byincrementally adjusting the spacing, shape and width of each elementresults in a filter passband centered at a desired frequency andexhibiting a flat frequency response through the passband. The length,width, spacing and proximity to ground of the resonators and thetransmission lines coupling the signal to and from the resonatorsdetermine the frequency response of the filter. If the printed circuitmanufacturing process is well controlled, the physical dimensions of theresonators will not vary and post fabrication tuning will not benecessary.

[0033] Rejection of the image channel component in the high IF signal130 can be improved by increasing the number of resonators in thefilter. However, adding additional resonators will increase the signalinsertion loss and the physical size of the filter.

[0034] In this invention, high IF signal 130 passes from the filterinput 202 through the input capacitor 204, the first intercoupler 210,and the second intercoupler 216. The bypass line input coupler 208couples a bypass line signal 230 from the high IF signal 130 and feedsthe delayed signal 230 forward through the bypass line 212 to the bypassline output coupler 218. The bypass line input coupler 208, the bypassline output coupler 218, the first intercoupler 210, and the secondintercoupler 216 each comprise a pair of coupled transmission lines. Acoupled transmission line can provide both inductive and capacitivecoupling. By carefully selecting the physical size and spacing of thebypass line 212, the input capacitor 204, the bypass line input coupler208, the bypass line output coupler 218, the first intercoupler 210, thesecond intercoupler 216, and the output capacitor 222, the bypass linesignal 230 is modified to be approximately equal in amplitude andopposite in phase to the image channel component of high IF signal 130.The output bypass line coupler 218 couples the bypass line signal 230with the high IF signal 130, substantially attenuating the image channelcomponent in the high IF signal 130 and sending the high filtered IFsignal 135 through output capacitor 222 to the filter output 224. Thisnovel feed forward feature increases image channel rejection by theprinted bandpass filter 200 without significantly increasing thefilter's insertion loss, physical size or component complexity.

[0035] Additional embodiments of the bandpass filter 200 can attenuatecomponents of the high IF signal 130 at frequencies other than the imagechannel frequency. This capability is built in to the bandpass filter200 and provides means to further improve the filtered high IF signal135 quality.

[0036]FIG. 3A illustrates a printed bandpass filter 300 which is aprinted version of the bandpass filter 200. Referring to FIG. 3A, theprinted bandpass filter 300 comprises an input capacitor 304 coupled tothe filter input 202, a bypass line input coupler 308, a first resonator306, and a first intercoupler 310. The first intercoupler 310 is coupledto a second intercoupler 316, and a second resonator 314. The secondintercoupler 316 is coupled to a bypass line output coupler 318, a thirdresonator 320, and an output capacitor 322. A bypass line 312 is coupledto the bypass line input coupler 308 and the bypass line output coupler318. The output capacitor 322 is coupled to the filter output 224.

[0037] The first resonator 306, the second resonator 314, and the thirdresonator 320 are coupled to a ground 360 (FIG. 3C) by vias 375 a, 375b, and 375 c. In the present embodiment, the via 375 is a plated-throughhole, electrically connecting portions of the printed filter 300 to theground 360. One of skill in the art will understand that additionalmethods of coupling components in a printed circuit board are availableand their use does not affect the invention described herein.

[0038] The input capacitor 304 and the output capacitor 322 are printedfinger capacitors. The bypass line input coupler 308, the bypass lineoutput coupler 318, the first intercoupler 310, and the secondintercoupler 316 are electromagnetically coupled segments of microstriptransmission line. The bypass line input coupler 308, the bypass lineoutput coupler 318, the first intercoupler 310, and the secondintercoupler 316 form a distributed transmission line implementation ofthe weak capacitive couplings used in a lumped-element LC bandpassfilter. Herein, weak capacitive coupling is the capacitance present inthe coupled transmission lines.

[0039] In one embodiment this capacitance is on the order of 0.2 pF.However, other values of capacitance could be used as will be understoodby one of ordinary skill in the relevant art.

[0040] In the printed filter 300, the ground 360 is located beneath thefilter 300. The ground 360 provides the return path necessary for bypassline input coupler 308, the bypass line output coupler 318, the firstintercoupler 310, the second intercoupler 316, the first resonator 306,the second resonator 314, and the third resonator 320 to function asmicrostrip transmission lines. Additional embodiments of this invention,with or without the ground 360 under the bandpass filter 200, can beimplemented using coplanar waveguide transmission lines.

[0041] The bypass line 312 is a microstrip transmission line. The firstresonator 306, the second resonator 314, and the third resonator 320each comprise a microstrip transmission line that is shorted at one end.The shorted transmission line presents an open circuit to a signalapplied at a resonant frequency.

[0042] Shorting each resonator reduces the resonant length from one-halfto one-quarter the wavelength of the desired resonant frequency. Theone-quarter wavelength microstrip transmission lines present anopen-circuit at a resonant frequency, and together with the inherentweakly capacitive couplings are equivalent to a parallel LC tank circuitat the resonant frequency. To reduce the physical size of printedbandpass filter 300, the resonators are printed in the form of spiralson the substrate. Ideally, coupling between adjacent portions of thesame resonator does not occur and the printed spiral is exactlyone-quarter wavelength, of the desired resonant frequency, in size.Unfortunately, there is electromagnetic coupling between adjacent turnsin each resonator spiral. To compensate for the undesired self coupling,the length of each resonator spiral is adjusted to become an opencircuit through the desired passband. Therefore, in the presentembodiment, the first resonator 306, second resonator 314, and the thirdresonator 320 are approximately one quarter wavelength in length. In thepresent invention, a quarter-wavelength resonator is a resonator,coupled to ground on one end and becoming an open circuit at the chosenresonant frequency.

[0043] The bypass line input coupler 308 comprises an outer segment ofthe printed trace forming the first resonator 306 and a segment of theprinted trace forming the bypass line 312. A signal in the firstresonator 306 is coupled to the bypass line 312 by the electromagneticcoupling between these parallel trace segments of the first resonator306 and the bypass line 312. Similarly, the bypass line output coupler318 comprises an outer segment of the printed trace forming the thirdresonator 320 and a section of the printed trace forming the bypass line312. The first intercoupler 310 comprises another outer segment of thetrace forming the first resonator 306 and an outer segment of the traceforming the second resonator 314. And, the second intercoupler 316comprises an outer segment of the trace forming the second resonator 314and an outer segment of the trace forming the third resonator 320.

[0044] The physical characteristics, trace width, length, and spacing,of the bypass line 312, the input capacitor 304, the bypass line inputcoupler 308, the bypass line output coupler 318, the first intercoupler310, the second intercoupler 316, and the output capacitor 322 areselected to cause the bypass line signal 230 to be approximately equalin amplitude and opposite in phase to the image channel component ofhigh IF signal 130. In additional embodiments of the present invention,the bypass line signal

[0045] Coupling effects between segments of the spirals are minimizedthrough adequate spacing, and the residual effects are modeled withfinite-element electromagnetic simulation software, such as IE3D (ZelandSoftware), SONNET (SONNET Software), Microwave Office (Applied WaveResearch) and Ensemble and HFSS (ANSOFT Corp., Pittsburgh, Pa.).

[0046] The physical arrangement of transmission lines in relation toeach other and to electrical ground, determines whether the transmissionline is classified as a coplanar waveguide or as a microstriptransmission line. Either type of transmission line can be used in thepresent invention to achieve the size, simplicity and performancebenefits discussed above.

[0047] Impedance and electrical length determine the properties of aquarter-wavelength resonator. Using a precision substrate material withwell-controlled electrical and mechanical properties, i.e., dielectricconstant, thickness, and dimensional stability, allows the circuitdesigner to control the filter's electrical characteristics verytightly. In one embodiment, the substrate material chosen is GML-1000(GIL Technologies, Collierville, Tenn.). Persons of skill in the artwill recognize additional substrate materials that can be substituted toprovide equivalent mechanical and electrical properties.

[0048] Controlling the substrate's electrical and mechanical propertiesmakes the filter less susceptible to variations in the manufacturingprocess. It helps ensure repeatable behavior in the intercouplersections, the bypass line input coupler and bypass line output couplersections, and in the bypass line itself. Repeatability means thevariations in electrical characteristics found in a lumped elementbandpass filter are not present in the printed bandpass filter. As aresult, there is no need to tune the present invention afterfabrication. Eliminating the necessity to tune the filter betweenmanufacture and installation is a significant reduction in manufacturingexpense.

[0049] Resonator to resonator coupling is accomplished by placing shortsegments of the resonators in close proximity to each other. Thistechnique is also used to couple the bypass line 312 in the printedfilter 300 using bypass line input coupler 308 and bypass line outputcoupler 318. Coupling the filter input 202 and the filter output 224 tothe printed filter 300 requires stronger coupling than can be achievedwith transmission lines. Therefore, finger capacitors are used for theinput capacitor 304 and the output capacitor 322. The electricalproperties of these capacitors are also well controlled if the substrateelectrical and mechanical properties are well controlled.

[0050]FIG. 3B illustrates a differential bandpass filter 350. Thedifferential bandpass filter 350 comprises the printed bandpass filter300 and a second printed bandpass filter 390. The second printedbandpass filter 390 comprises the mirror image of the printed bandpassfilter 300, a second filter input 302 and a second filter output 324.The printed bandpass filter 300 and the second printed bandpass filter390 are printed on the same substrate in a manner to allow adifferential signal to be applied to the filter input 202 and the secondfilter input 302. The filtered differential signal is output at thefilter output 224 and the second filter output 324. The second printedbandpass filter 390 functions similarly to the printed bandpass filter300. The differential bandpass filter 350 enables a balanced signal tobe used in tuner 100. A balanced signal exhibits higher dynamic range,higher bandwidth, and lower pick-up and generation of interference noisethan an unbalanced signal.

[0051]FIG. 3B also illustrates the required trace widths, trace lengthsand trace spacings for an embodiment of the printed bandpass filter 300.The input capacitor 304 and the output capacitor 322 have calculatedvalues of 0.19 pF. The differential bandpass filter 350 has a Bandwidthof 1199˜1240 M, an insertion loss of −3 dB, a differential input anddifferential output impedance of 200 ohms and an image rejection of >40dB. FIG. 3B also illustrates one limitation on the ground 360, printedon the opposite side of a substrate 370 (FIG. 3C). The ungrounded zones385 a, 385 b, 385 c, and 385 d are areas where the ground 360 isexcluded to allow the high IF signal 130 and the filtered high IF signal135 to be coupled to and from the differential bandpass filter 350.

[0052]FIG. 3C illustrates a three-dimensional view of a printed filterassembly 380. Referring to FIG. 3C, the filter assembly 380 comprises aprecision substrate 370, the differential bandpass filter 350 printed onthe upper side of the precision substrate 370, and a ground 360 printedon the lower side of the precision substrate 370. The ground 360 is ametal pattern placed on the precision substrate 370 opposite the sidethe differential bandpass filter 350 is placed. One method of couplingthe ground 360 to the differential bandpass filter 350 is by the via375. Referring to FIG. 3A, the first resonator 306, the second resonator314 and the third resonator 320 are coupled to the ground 360 by vias375 a, 375 b, and 375 c respectively.

[0053] The proximity of the differential bandpass filter 350 and theground 360 can cause a waveguide like effect in the precision substrate370. This effect results in some portion of high IF signal 130 bypassingthe differential bandpass filter 350 and increasing the out of bandsignal strength at filter output 224 and 324. This effect isundesirable.

[0054]FIG. 3C also illustrates a plurality of vias 375 coupled betweenthe ground 360, a first blocking strip 376 and a second blocking strip377. The vias 375, the blocking strips 376 and 377 and the ground 360act as shields to greatly reduce the IF signal leaking into thesubstrate.

[0055]FIGS. 4A and 4B illustrate the differential bandpass filter 350connections used when printed filter assembly 380 is mounted on a secondprinted circuit board 410 (FIG. 4B). The second printed circuit board410 incorporates elements of the tuner 100, and possibly other circuits,which are part of a larger assembly, including the tuner 100. The filterinput 202, the filter second input 302, the filter output 224 and thesecond filter output 324 are connected to vias 375 a, 375 b, 375 c, and375 d respectively. The vias 375 are connected to the bottom (opposite)side of the precision substrate 370.

[0056]FIG. 4B further illustrates a land pattern 420 used to connect thedifferential bandpass filter 350 to the second printed circuit board410. Coupling the printed filter assembly 380 to the second printedcircuit board 410 eliminates the need to use the relatively expensiveprecision substrate 370 for the second printed circuit board 410.

[0057] Referring to FIG. 4B, the via 375 a is coupled between the filterinput 202 and the land pattern 420 a. When the printed circuit assembly380 is landed on the second printed circuit board 410, the land pattern420 a couples the positive component of high IF signal 130 from thesecond printed circuit board 410 to the filter input 202. The via 375 bis coupled between the second filter input 302 and the land pattern 420b. When landed, the land pattern 420 b couples the negative component ofhigh IF signal 130 from the second printed circuit board 410 to thesecond filter input 302. In a similar manner the via 375 c is coupledbetween the filter output 224 and a corresponding (not shown) landpattern, and the via 375 d is coupled between the second filter output324 and a corresponding land pattern (not shown). Additional vias 375and land patterns 420 are used as necessary to route additionalconnections between the printed filter assembly 380 and the secondprinted circuit board 410.

[0058] The land pattern 420 and the ground 360 coexist on the same sideof the precision substrate 370. The land pattern 420 and the ground 360are separated where necessary to couple a signal carrying element fromthe differential bandpass filter 350 to the land pattern 420. The landpattern 420 and the ground 360 are coupled where necessary to connectthe ground 360 to the second printed circuit board 410.

[0059] In one embodiment, FR-4 is used for the printed circuit board410. The precision substrate 370 has a slightly larger coefficient ofthermal expansion than the printed circuit board material (FR-4). Thedifference in thermal expansion coefficient causes repetitive thermalstresses to be applied to the solder connections between the printedfilter assembly 380 and the second printed circuit board 410.

[0060]FIG. 5 illustrates an apparatus 500 for mitigating thermal cyclingstress while maintaining the necessary electrical coupling between thedifferential bandpass filter 350 and the second printed circuit board410. The apparatus 500 comprises a cup 515 coupled to a riser 510, toabase 525, and to a lower element 520. One example of the apparatus 500is surface mount pin (model 34AC) by NAS Interplex, Flushing, N.Y.

[0061] In this example, the apparatus is connected to the filter input202. Additional apparatus 500 can be connected, as described below, toprovide thermal stress relief for any connection between the printedfilter assembly 380 and the second printed circuit board 410. Theseconnections comprise the second filter input 302, the filter output 224,the second filter output 324, and ground 360.

[0062] The cup 515 is connected to the filter input 202 and the lowerelement 520 is connected to the land pattern 420. The cup 515 is alsocoupled to via 375 which is coupled to land pattern 420. The apparatus500 and the via 375 ensure a good electrical connection is maintainedbetween the filter input 202 and the second printed circuit board 410.Any differential thermal expansion is absorbed by deflection of theriser 510 vice deflection of an affected solder connection.

[0063] Designing the Printed Bandpass filter

[0064] The detailed design of the filter is accomplished using numericaloptimization techniques. First, the structure of the filter is describedin terms of coupled microstrip lines and input and output couplingcapacitances using the RF circuit simulator MMICAD (Optotek Ltd.,Kanata, Ontario, Canada), or any equivalent microwave circuit simulatorwith an optimizer. Lengths of the resonator and coupling sections,spacing of the coupled sections, input and output coupling capacitors,and length of the bypass line are variables to be optimized (althoughapproximate initial values were specified as a starting point). When anacceptable design was obtained using MMICAD, the filter's physicalparameters were refined using electromagnetic finite-element simulation,as described above.

[0065]FIG. 6 illustrates the steps of a method for printed bandpassfilter design 600. In step 610, the variable filter design parametersused in the design are selected. In step 620, printed bandpass filterperformance is simulated. In step 640, if simulated printed bandpassfilter performance is equal to the filter design goal performance, step630 is performed. If printed bandpass filter simulated performance isdifferent from filter design goal performance, step 650 is performed. Instep 630, the filter design is complete. In step 650, the filter designparameters are incrementally varied in a manner to cause the simulatedprinted filter performance to approach the design goal performance.Steps 620, 630, and 640 are repeated until the simulated filterperformance is equal to the design goal performance.

[0066] Conclusion

[0067] Example embodiments of the methods, systems, and components ofthe present invention have been described herein. As noted elsewhere,these example embodiments have been described for illustrative purposesonly, and are not limiting. Other embodiments are possible and arecovered by the invention. Such embodiments will be apparent to personsskilled in the relevant art based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A bandpass filter, comprising: a plurality of resonators that are electromagnetically coupled to each other, each resonator having a terminal coupled to a ground; a bypass line in parallel with said plurality of resonators, said bypass line having a bypass line input coupled to a first resonator of said plurality of resonators and a bypass line output coupled to a second resonator of said plurality of resonators; an input, coupled to said first resonator; and an output coupled to said second resonator.
 2. The bandpass filter of claim 1, wherein said resonators are quarter wavelength transmission lines.
 3. The bandpass filter of claim 2, wherein said quarter wavelength transmission lines are microstrip transmission lines, said microstrip transmission lines printed in a spiral pattern.
 4. The bandpass filter of claim 3, further comprising: an input capacitor coupled between said input and said first resonator; and an output capacitor coupled between said output and said second resonator.
 5. The bandpass filter of claim 4, wherein said input capacitor and said output capacitor are printed finger capacitors.
 6. The bandpass filter of claim 4, further comprising: a bypass line input coupler, coupled between said bypass line and said first resonator; a bypass line output coupler, coupled between said bypass line and said second resonator.
 7. The bandpass filter of claim 6, wherein said plurality of resonators includes a third resonator coupled between said first resonator and said second resonator.
 8. The bandpass filter of claim 7, further comprising: a precision substrate, wherein said plurality of resonators, said bypass line, said input capacitor, said output capacitor, said bypass line input coupler, and said bypass line output coupler are printed on said precision substrate.
 9. The bandpass filter of claim 1, wherein an input impedance and an output impedance are a desired value.
 10. A bandpass filter comprising: an input coupled to an input capacitor; an output coupled to an output capacitor; a first resonator coupled to a ground, said input capacitor, a first intercoupler and a bypass line input coupler; a second resonator coupled to said ground, a second intercoupler, a bypass line output coupler, and said output capacitor a third resonator coupled to said ground, said first intercoupler, and said second intercoupler, wherein said first resonator, said second resonator and said third resonator are electromagnetically coupled quarter wavelength transmission lines; a bypass line coupled between said input bypass line coupler and said output bypass line coupler; wherein said bypass line causing improved image channel signal rejection at said output; and a precision substrate, wherein said first resonator, said second resonator, said third resonator, said bypass line, said input capacitor, said output capacitor, said bypass line input coupler, and said bypass line output coupler are printed on said precision substrate.
 11. The bandpass filter of claim 10, wherein said input capacitor and said output capacitor are printed finger capacitors.
 12. A differential bandpass filter, comprising: a plurality of resonators that are electromagnetically coupled to each other, each resonator having a terminal coupled to a ground; a first bypass line; in parallel with said plurality of resonators, said bypass line having a bypass line input coupled to a first resonator and a bypass line output coupled to a second resonator; a first input, coupled to said first resonator; a first output coupled to said second resonator. a second plurality of resonators that are electromagnetically coupled to each other, each resonator having a terminal coupled to said ground; a second bypass line; in parallel with said second plurality of resonators, said second bypass line having a second bypass line input coupled to a third resonator and a second bypass line output coupled to a fourth resonator; a second input, coupled to said third resonator; and a second output coupled to said fourth resonator.
 13. A bandpass filter, comprising: an input; an output; filtering means, coupled between said input and said output, for passing a desired frequency band to said output; and frequency attenuating means, coupled between said input and said output, for attenuating a frequency component of said signal.
 14. The bandpass filter of claim 13, further comprising: shielding means for shielding the filter periphery, wherein said shielding reduces the amount of said signal bypassing at least one of said filtering means and said frequency attenuating means.
 15. A double conversion tuner, comprising: a tuner input; a variable gain amplifier, coupled to said tuner input; a first mixer coupled to a first local oscillator and said low noise amplifier; a printed bandpass filter, coupled between said first mixer and a second mixer, including: a plurality of resonators that are electromagnetically coupled to each other, each resonator having a terminal coupled to a ground; a bypass line; in parallel with said plurality of resonators, said bypass line having a bypass line input coupled to a first resonator and a bypass line output coupled to a second resonator; a bandpass filter input, coupled to an output of said first mixer; a bandpass filter output coupled to an input of said second mixer; a second local oscillator, coupled to said second mixer; a second IF bandpass filter coupled to said second mixer and a variable gain amplifier; and a tuner output, coupled to said variable gain amplifier.
 16. The double conversion tuner of claim 15, wherein said printed bandpass filter is a differential bandpass filter.
 17. The bandpass filter of claim 7, further comprising: a first intercoupler that weakly couples said first resonator to said third resonator; and a second intercoupler that weakly couples said third resonator to said second resonator. 